1. Field of the Invention
The present invention relates to a reflector having, on a reflector body, a surface which is resistant to mechanical and chemical attack and exhibits high total reflectivity. The invention relates also to a process for its manufacture and the use of such reflectors with a resistant surface layer.
2. Background Art
Known is the production of bright finish strips e.g. of high purity aluminium or AlMg alloys based on aluminium having a purity of 99.8% and higher, such as e.g. 99.9%, and roll surfaces that produce diffuse or directionally reflected light depending on the application. To increase the directional reflectivity (degree of brightness) of such strips, it is also known to brighten the strips chemically or electrolytically, then to create a e.g. 1.5 μm thick protective layer by anodic oxidation.
Anodising processes are chemical treatments which suffer the disadvantage of requiring considerable precaution in order to avoid contamination of the environment The extent of these precautions increases with increasing thickness of the oxide layer.
The known processes have the further disadvantage that high purity, expensive bright-finish alloys based on aluminium of very high purity have to be employed. As a result of the anodising process, the degree of reflectivity of the surface, and with that both the total reflectivity and the directional reflectivity is lowered due to absorption and diffuse scattering of light, in particular in the oxide layer. This represents a loss in energy.
Known from EP-A-0 495 755 are objects with surfaces of aluminium which are suitable for the precipitation of layer systems from the gas phase onto these surfaces. Anodising the surface is dispensed with and the layer system described involves e.g. a bonding layer, such as a ceramic layer, a light-reflecting layer, such as a metallic layer e.g. of aluminium, and one or more transparent protective layers of metallic compounds. Such layer systems exhibit a high degree of reflectivity and the disadvantages of anodising are avoided. Such a layer system, however, suffers the disadvantage that the surfaces are very sensitive to physical attack, such as mechanical or chemical attack e.g. by corrosive media.
EP-A-0 568 943 describes the precipitation of a reflective layer on the basis of aluminium or an aluminium alloy and a gel film which has been deposited on the aluminium by means of a sol-gel process. This is also a possible way of arriving at reflective aluminium materials without having to employ anodising; the layered structure described in EP-A-0 568 943 is, however, not resistant to mechanical effects and corrosion to the extent desired.